1. Technical Field
This invention relates to an optical disc. The optical disc includes optical recording discs such as, for example, magneto-optical discs, phase change type optical recording discs, and write-once optical recording discs as well as read-only optical discs.
2. Background Art
In general, optical discs have information-carrying means in the form of a recording layer or a row of pits on one surface of a substrate. For writing and reading purposes, a light beam, typically a laser beam is directed to the information-carrying means from the rear surface of the substrate. The substrate is generally made of a transparent material for this reason. In the prior art, resin substrates are generally used in order to reduce the disc weight and facilitate formation of grooves and pits for tracking and other purposes.
Over the recent past, active research has been made on optical discs that carry moving picture digital information. To this end, it is essential to increase the capacity and data transfer rate of optical discs and hence, the diameter and revolution of optical discs.
However, resin substrates are difficult to manufacture to uniform quality. Especially when the resin substrates have a large diameter, they tend to be non-uniform, allowing birefringence and other factors to adversely affect writing and reading performance. Additionally, the resin substrates are less resistant to heat and moisture and undergo deformation and deterioration in a hot humid atmosphere. Also since resin substrates with a large diameter are less stiff, high-speed revolution at 1,800 rpm or higher, especially 3,000 rpm or higher causes axial runouts, resulting in a significant increase of focusing and tracking errors.
Under these circumstances, attention is paid to glass substrates having high mechanical strength. The glass substrates, however, are likely to fail, especially near their corner, upon receipt of impact as by accidental dropping or hitting. Although substrates of chemically strengthened glass having higher mechanical strength are known, they can be crushed into scattering chips. Some optical discs are received in cartridges, but still can be broken as a result of vibration of the disc within the cartridge.
Japanese Patent Application Kokai (JP-A) No. 203409/1994 discloses a disc-shaped recording medium comprising a disc-shaped glass substrate and a disc-shaped resin substrate joined thereto wherein the resin substrate has a larger diameter than the glass substrate. The periphery of this recording medium has a cross section as shown in FIG. 5. A resin substrate 4 having a larger diameter is joined to a glass substrate 2 with an adhesive layer 5 while a recording layer 3 on the glass substrate 2 is disposed inside. However, this structure cannot fully prevents a failure of the corner of the glass substrate when impact is applied thereto from an oblique direction.
Japanese Utility Model Application Kokai No. 38340/1982 discloses an optical information recording disc having a glass substrate received in a plastic holder frame. This optical information recording disc is shown in FIG. 6 as comprising a glass substrate 2, a metallization 31 coated on the glass substrate 2 and selectively etched by laser processing in accordance with external signals for recording information, a plastic holder frame 7 having a concave for receiving the metallized glass substrate 2, and a protective plate 10 removably fitted in the plastic holder frame 7 above the concave for covering the glass substrate 2. The protective plate 10 is rigidly secured to the frame 7 by a stop ring 8. This optical information recording disc requires to remove the protective plate 10 upon writing and attach the protective plate 10 again upon reading. This structure intends to prevent direct contact of the recording surface with the ambient atmosphere and reduce the weight, but not to prevent a failure of the glass substrate near its corner. In fact, this optical information recording disc is less effective for preventing a failure of the glass substrate 2 near its periphery since the periphery of the glass substrate 2 is in close fit with the plastic holder frame 7.